This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A057010 #12 Aug 10 2019 05:34:25 %S A057010 1,26,604,14120,334576,7987616,191318464,4588288640,110090411776, %T A057010 2641931680256,63404394241024,1521689370306560,36520413978750976, %U A057010 876488875160477696,21035724442850934784,504857317670233210880 %N A057010 Number of conjugacy classes of subgroups of index 4 in free group of rank n. %D A057010 R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.13(c), pp. 76, 112. %H A057010 J. H. Kwak and J. Lee, <a href="https://doi.org/10.1002/(SICI)1097-0118(199610)23:2<105::AID-JGT1>3.0.CO;2-X">Enumeration of connected graph coverings</a>, J. Graph Th., 23 (1996), 105-109. %H A057010 J. H. Kwak and J. Lee, <a href="https://web.archive.org/web/20061002144237/http://com2mac.postech.ac.kr/Lecture/Lec-1.pdf">Enumeration of graph coverings and surface branched coverings</a>, Lecture Note Series 1 (2001), Com^2MaC-KOSEF, Korea. See chapter 3. %H A057010 V. A. Liskovets, <a href="https://doi.org/10.1023/A:1005950823566">Reductive enumeration under mutually orthogonal group actions</a>, Acta Applic. Math., 52 (1998), 91-120. %F A057010 G.f.: x(1-12x)/((1-6x)(1-8x)(1-24x)). %F A057010 a(n) = 24^(n-1)+8^(n-1)-6^(n-1). %o A057010 (PARI) a(n)=if(n<0,0,24^(n-1)+8^(n-1)-6^(n-1)) %Y A057010 Cf. A057004-A057013. %K A057010 nonn %O A057010 1,2 %A A057010 _N. J. A. Sloane_, Sep 09 2000 %E A057010 More terms from Francisco Salinas (franciscodesalinas(AT)hotmail.com), Dec 25 2001